In general, a magnetic sensor which uses a Hall element is not only used as a proximity sensor, linear position sensor, rotational position sensor, etc. as a sensor for detecting positional information of a magnet, but is also widely utilized as a current sensor which detects the magnetic field which is created by the current flowing through a current conductor so as to measure the amount of current which flows through the current conductor by contactless means.
In particular, in a current sensor which is utilized for detecting an inverter current of a motor, it is demanded to detect the inverter current, which is switched at a high frequency, with high precision for the purpose of increasing the efficiency of motor control.
This type of Hall element has a magneto-electric conversion function which generates a Hall electromotive force signal in accordance with the intensity of the magnetic field which is input to the Hall element, so is being broadly used as a magnetic sensor. However, a Hall element has an offset voltage and this offset leads to a not-zero finite voltage being output even in a zero magnetic field state, that is, in the absence of a magnetic field.
Therefore, in a magnetic sensor which utilizes a Hall element, there is the method of driving the Hall element, known as the “spinning current method” or “connection commutation method”, for the purpose of cancelling out the offset voltage of the Hall element. This method comprises the operation of periodically switching between the position of a terminal pair for injecting a drive current to the Hall element and the position of a terminal pair for detecting the Hall electromotive force signal in accordance with a clock called a “chopper clock” (for example, see Non-Patent Document 1).
The spinning current method designed for the cancellation of this offset voltage can be realized by using a switch circuit in a CMOS semiconductor circuit as well, so a Hall electromotive force detection circuit for realizing a high precision magnetic sensor is generally provided with a switch circuit for realizing the spinning current method.
Moreover, the Hall electromotive force signal which is generated in a Hall element is generally weak, so for the purpose of amplifying this Hall electromotive force signal, a Hall electromotive force signal detection circuit is a circuit which includes a signal amplification circuit. Here, when this signal amplification circuit has a finite offset voltage, the offset voltage of the signal amplification circuit must also be cancelled.
Under these circumstances, for a Hall electromotive force signal detection circuit which detects the Hall electromotive force which is generated by a Hall element and amplifies the signal, the circuit configuration of a signal amplification circuit which uses a current feedback type amplification circuit suitable for the combination with the spinning current method of a Hall element is known. In this current feedback type amplification circuit, a circuit configuration which uses a chopper clock to modulate the offset voltage of the amplification circuit to the frequency of the chopper clock is a circuit configuration which is generally known as a “chopper amp”.
As explained above, if using a Hall electromotive force detection circuit which combines the circuit configuration of a circuit which realizes the spinning current method in the Hall element and a chopper amp in a signal amplification circuit, it is known that it is possible to modulate both the offset voltage of the Hall element and the offset voltage of the signal amplification circuit at the frequency of the chopper clock (for example, see Patent Document 1 and Non-Patent Document 2).
Below, the cancellation of the offset of a Hall element by the spinning current method will be explained.
FIGS. 1A and 1B illustrate the detection of a Hall electromotive force when changing the direction of the drive current which biases the Hall element between 0 degree and 90 degrees each time the phase of the chopper clock switches between the binary values of φ1, φ2. FIG. 1A illustrates the detection of the Hall electromotive force when the phase of the chopper clock is φ1 and the direction of the drive current of the Hall element is 0 degree. FIG. 1B illustrates the detection of the Hall electromotive force when the phase of the chopper clock is φ2 and the direction of the drive current of the Hall element is 90 degrees. Note that, the Hall element is modeled as a four-terminal element which is comprised of four resistors and is driven by a constant current.
In FIGS. 1A and 1B, when the direction of the drive current of the Hall element is switched between 0 degree and 90 degrees, the voltage signals Vhall(φ1) and Vhall(φ2) measured at the Hall element are, as shown in Formula 1, expressed as the sum of the Hall electromotive force signal Vsig(B) in accordance with the magnetic field B which is detected by the magnetic sensor using the Hall element and the offset voltage Vos(Hall) of the Hall element.
Here, by periodically switching the direction of the bias current of the Hall element in accordance with the chopper clock between 0 degree and 90 degrees, it is possible to switch the polarity of the Hall electromotive force signal Vsig(B) corresponding to the magnetic field being detected between inverted/noninverted, so it is possible to modulate the Hall electromotive force signal Vsig(B) corresponding to the magnetic field being detected to the frequency f_chop of the chopper clock. On the other hand, regarding the DC offset voltage Vos(Hall) of the Hall element, when the direction of the drive current of the Hall element is switched between 0 degree and 90 degrees, the DC offset has the same polarity, so Vos(Hall) is not modulated in frequency domain to the chopper clock.
                                              ⁢                              Signal            ⁢                                                  ⁢            generated            ⁢                                                  ⁢            at            ⁢                                                  ⁢            Hall            ⁢                                                  ⁢            element                    ⁢                                          ⁢                      {                                                                                                      Vhall                      ⁡                                              (                                                  ϕ                          ⁢                                                                                                          ⁢                          1                                                )                                                              =                                                                  +                                                  Vsig                          ⁡                                                      (                            B                            )                                                                                              +                                              Vos                        ⁡                                                  (                          Hall                          )                                                                                                                                                          (                                                                  when                        ⁢                                                                                                  ⁢                        chopper                        ⁢                                                                                                  ⁢                        clock                                            =                                              φ                        ⁢                                                                                                  ⁢                        1                                                              )                                                                                                                                          Vhall                      ⁡                                              (                                                  ϕ                          ⁢                                                                                                          ⁢                          2                                                )                                                              =                                                                  -                                                  Vsig                          ⁡                                                      (                            B                            )                                                                                              +                                              Vos                        ⁡                                                  (                          Hall                          )                                                                                                                                                          (                                                                  when                        ⁢                                                                                                  ⁢                        chopper                        ⁢                                                                                                  ⁢                        clock                                            =                                              φ                        ⁢                                                                                                  ⁢                        2                                                              )                                                                                                          Formula        ⁢                                  ⁢        1            
As explained above, when performing an operation of switching the direction of the drive current of a Hall element in accordance with a chopper clock between 0 degree and 90 degrees, the signal Vhall which is generated at the Hall element becomes a waveform such as shown in FIGS. 2A to 2D. Moreover, the spectrum of the signal which is generated at the Hall element becomes the spectrum such as shown in FIG. 3, so it will be understood that the Hall electromotive force signal Vsig(B) according to the magnetic field which is being detected and the offset voltage Vos(Hall) of the Hall element are separated in the frequency domain. This is the principle of the cancellation of the offset by the spinning current method.
In the explanation given above, the direction of the drive current of a Hall element with four terminals is switched between 0 degree and 90 degrees. However, it should be noted that the separation of the Hall electromotive signal and the offset voltage in the frequency domain is also possible when the direction of the drive current of the Hall element is periodically selected among 0 degree, 90 degrees, 180 degrees, and 270 degrees.
The Hall electromotive force signal Vsig(B) which is modulated by the frequency f_chop of the chopper clock as explained this way is demodulated using the chopper clock in the later stage of the Hall electromotive force signal detection circuit. As a result of this demodulation operation, the signal spectrum at the output signal of the Hall electromotive force signal detection circuit becomes such as shown in FIG. 7. Therefore, as will be understood from the signal spectrum of FIG. 7, the waveform of the output signal of the Hall electromotive force signal detection circuit which is seen in the time domain becomes the waveform of the Hall electromotive force signal Vsig(B) according to the magnetic field which is being detected plus, superposed, the ripple which is generated by modulating the offset voltage Vos(Hall) of the Hall element to the frequency f_chop of the chopper clock.
In order to realize a high precision Hall electromotive force signal, the occurrence of ripple at the output signal of this Hall electromotive force signal detection circuit must be canceled. Therefore, as one circuit system for cancelling this occurrence of the ripple, there is a circuit system which utilizes a feedback from the output signal of the Hall electromotive force signal detection circuit.
As explained above, for a Hall electromotive force signal detection circuit of the type which uses feedback to cancel the occurrence of the ripple in the output signal after signal amplification, for example, there is the one of Patent Document 1. As a circuit system which cancels the effect of an error signal (offset) in the output signal which takes binary values, for example, there is the one of Patent Document 2. In this Patent Document 2, a circuit configuration which uses an AD converter, digital circuit, and DA converter in a feedback circuit for cancelling the occurrence of the ripple is shown.